Honda's are very reliable motors and you need not worry too much. I would check the thermostat area if the motor has seen salt, as you can get a build up of crystallized salt deposits there, and it can restrict the cooling water flow. A spare set of plugs is fine, but not much good without the tools to change them. The plug size is 18mm and not in most tool kits, and hard/impossible to get with a wrench. If you have the Honda tool kit, it comes with this wrench and a starter rope should yours fail. There are also cotter pins and shear pins in a rubber holder attached to the motor opposite the tiller.
I don't know the size of your boat, but my 33 year old Honda 7.5 starts first pull, and pushes my boat at hull speed with almost zero throttle. Fuel use is nil at hull speed.